1. Field of the Invention
The present invention relates to a glass panel for a cathode ray tube used mainly for receiving TV broadcasting and for industrial equipments.
2. Discussion of Background
As shown in FIG. 3 a cathode ray tube 1 has a glass bulb 2 which is generally composed of a glass panel 3 for displaying a picture image, a funnel portion 4 mounted thereon a deflection coil and a neck portion 5 for housing an electron gun 17.
In FIG. 3, reference numeral 6 designates a panel skirt portion, numeral 7 designates a face portion on which a picture image is displayed, numeral 8 designates an anti-implosion band for providing strength against a mechanical shock, numeral 9 designates a blend R portion connecting the face portion to the skirt portion, numeral 10 designates a sealing portion at which the glass panel 3 and the funnel portion 4 are sealed with a solder glass or the like, numeral 12 designates a fluorescent layer for emitting fluorescence by irradiating electron beams, numeral 13 designates an aluminum layer for reflecting forwardly the fluorescence at the fluorescent layer, numeral 14 designates a shadow mask which specifies positions of electron beams on a fluorescent substance, numeral 15 designates a stud pin for fixing the shadow mask 14 to an inner surface of the skirt portion 6,and numeral 16 designates an inner conductive coating which prevents the shadow mask 14 from being charged to a high potential by the electron beams and which grounds electric charges to the outside. A symbol A indicates a tube axis connecting the central axis of the neck portion 5 to the center of the glass panel 3. The fluorescent layer 12 is formed on an inner surface of the glass panel to thereby form a screen. The screen is substantially in a rectangular shape constituted by four side lines which are in substantially parallel to a long axis and a short axis which cross at a right angle to the tube axis A at the center of the tube axis A.
The inside of the cathode ray tube is kept under a highly vacuumed condition because a picture image is displayed by irradiating electron beams on the face portion. The cathode ray tube has an asymmetric shape unlike a spherical shape and suffers a difference of 1 atmospheric pressure between-the outside and inside of the glass panel. Accordingly, there exists a high deformation energy and is under an unstable condition. Under such condition, when a crack is generated in the glass panel, the crack tends to extend rapidly to release the high deformation energy whereby there causes a large scale destruction of the glass panel wherein a number of cracks expand into the entire of the panel.
In particular, when the rate of crack extension is high in a case such as the destruction by a mechanical shock, the glass panel is broken instantaneously. In this case, there cause an implosive shrinkage phenomenon and the reaction thereof which result an intensive implosion wherein a large amount of glass pieces are scattered. In many cases, a reinforcing band 8 is attached to a side surface of the glass panel 3 to protect a user from the implosion and to suppress the extension of the cracks and the breakage of the bulb body.
In recent years, the face portion of the g lass panel tends to be flattened in order to improve the visibility of the cathode ray tube. With this, there is a tendency that the asymmetry of the cathode ray tube derived from its structure becomes remarkable whereby an implosion can be occurred.
In the cathode ray tube having the above-mentioned structure, when a mechanical shock is given to a diagonally opposing portion or its vicinity, which has structurally a high rigidity, a change of stress in terms of time generated in the diagonally opposing portion or its vicinity is sharp and large whereby a rate of occurrence of cracks is high. Further, since the speed of the extension of cracks is high, an implosion phenomenon is further apt to occur. Accordingly, it may be necessary to increase the wall thickness of the face portion to thereby reduce the stress to be generated in order only to prevent the implosion which is generated when a shock is given to the diagonally opposing portion. In this case, the weight of the cathode ray tube is increased, which is the great problem in the cathode ray tube.